A method for transferring a layer known from the prior art, called SMART CUT®, comprises the following steps:                a) providing a support substrate,        b) providing a donor substrate,        c) forming an embrittlement region in the donor substrate so as to delimit a first portion and a second portion of the donor substrate on either side of the embrittlement region, the first portion of the donor substrate being intended to be added onto the support substrate, the second portion of the donor substrate being able to be recycled,        d) assembling the donor substrate on the support substrate,        e) fracturing the donor substrate along the embrittlement region, preferably by a heat treatment.        
After step e), the first portion of the donor substrate forms the layer transferred onto the support substrate, and is commonly called an “active layer.”
It is found from trial and error that the free surface of the active layer exhibits a high roughness, typically between 50 and 70 Å RMS, typically 60 Å RMS, which is mainly due to the propagation of the fracture front along the embrittlement region.
This surface roughness may require a significant number of steps for treating the free surface of the active layer in order to reduce its roughness so as to obtain a value of typically between a few Å RMS and 30 Å RMS. These treatment steps may, in particular, comprise anneal, oxidation and polishing steps. These treatment steps are notably detailed in the documents EP 0 986 826, EP 1 208 589 and EP 1 208 593.
A method known from the prior art, notably from U.S. Pat. No. 6,284,631, proposes forming a stress layer in the donor substrate, and forming the embrittlement region inside the stress layer so as to facilitate the fracture along the embrittlement region. According to this document, this prior art method would facilitate the propagation of the fracture front by reducing the energy level required for the fracture in the stress layer, by compression or by tension.
Even though such a prior art method would make it possible to reduce the fracture energy level and control the initiation of the propagation of the fracture front, this method does not make it possible to control the spatial dispersion of the propagation of the fracture front and, therefore, to significantly reduce the surface roughness of the active layer. To this end, U.S. Pat. No. 6,284,631 proposes limiting the energy or the stresses in the donor substrate in order to reduce the possibility of uncontrolled initiations of fracture fronts and reduce the surface roughness.
Consequently, such a prior art method is complex to implement since it is necessary to determine an energy for the stress layer that is sufficiently high to reduce the energy of the fracture and sufficiently low to reduce the possibility of uncontrolled initiations of fracture fronts and reduce the surface roughness.
Moreover, such a prior art method requires excellent control of the depth and of the thicknesses of the stress layer and of the embrittlement region in as much as the embrittlement region must, without fail, be formed inside the stress layer.